1. Field of the Invention
The present invention relates to an electrophotographic photoconductor (hereafter also referred to simply as “photoconductor”), to a method for producing the electrophotographic photoconductor, and to an electrophotographic apparatus. More particularly, the present invention relates to an electrophotographic photoconductor that is used in electrophotographic printers, copiers, fax machines and the like, to a method for producing the electrophotographic photoconductor, and to an electrophotographic apparatus.
2. Background of the Related Art
Generally, image forming apparatuses that rely on electrophotographic schemes, for instance printers, copiers, fax machines and the like, are provided with a photoconductor as an image carrier, a charging device that charges homogeneously the surface of the photoconductor, an exposure device that draws, on the surface of the photoconductor, an electrical image (electrostatic latent image) according to an image, a developing device that develops, with toner, the electrostatic latent image, to form thereby a toner image, and a transfer device that transfers the toner image to transfer paper. The electrophotographic apparatus is also provided with a fixing device for fusing, onto the transfer paper, the toner that has been transferred thereonto.
The photoconductors that are used on such image forming apparatuses vary depending on the concept of the apparatus. However, with the exception of inorganic photoconductors such as Se or a-Si, in large machines and high-speed machines, organic photoconductors (OPCs) in which an organic pigment is dispersed in a resin are widely used at present on account of their superior stability, cost and ease of use. These organic photoconductors are generally of negatively chargeable type, unlike inorganic photoconductor which are of positively chargeable type. One reason for this is that development of hole transport materials that afford a good hole transport function, in negatively-chargeable organic photoconductors, has been going on for a long time, whereas few electron transport materials having good electron transport capability have been developed in positively-chargeable organic photoconductors.
The negative charging process in negatively-chargeable organic photoconductors is problematic in that the amount of ozone generated due to negative-polarity corona discharge is far larger, about ten times, than that of positive polarity. This has an adverse effect on the photoconductor and on the usage environment. Accordingly, the ozone generation amount is curtailed by resorting to a contact charging scheme, such as roller charging or brush charging in the negative charging process. However, contact charging schemes have drawbacks, for instance, in being disadvantageous in terms of cost, as compared with positive-polarity contactless charging schemes, and also in terms of entailing unavoidable contamination of a charging member, and insufficient reliability. Contact charging schemes have also drawbacks when it comes to affording high image quality, since, for instance, it is difficult to achieve homogeneous surface potential in the photoconductor.
In order to solve these problems, high-performance positively-chargeable organic photoconductors are required that can be used effectively. Other advantages of positively-chargeable organic photoconductors, besides those that are inherent to positive charging schemes, as described above, include less transverse diffusion of carriers than in the case of negatively-chargeable organic photoconductors, and thus superior dot reproducibility (resolution and gradation properties), since the carrier generation position is generally close to the surface of a photoconductive layer. Accordingly, positively-chargeable organic photoconductors are being studied in fields where ever higher resolutions are sought after.
Positively-chargeable organic photoconductors have roughly four types of layer configuration, as described below, for which various conventional configurations have been proposed. The first configuration is that of a function-separated photoconductor having a two-layer configuration in which a charge transport layer and a charge generation layer are stacked, in this order, on a conductive support, see, for instance, Japanese Examined Patent Publication H05-30262 (Patent literature 1) and Japanese Patent Application Publication No. H04-242259 (Patent literature 2). A second configuration is that of a function-separated photoconductor having a three-layer configuration in which a surface protective layer is stacked on the above two-layer configuration, see, for instance, Japanese Examined Patent Publication H05-47822 (Patent literature 3), Japanese Examined Patent Publication H05-12702 (Patent literature 4), and Japanese Patent Application Publication No. H04-241359 (Patent literature 5). A third configuration is that of a function-separated photoconductor having a two-layer configuration, reverse to that of the first configuration, i.e. a configuration in which a charge generation layer and a charge (electron) transport layer are reversely stacked, in this order, see, for instance, Japanese Patent Application Publication No. H05-45915 (Patent literature 6) and Japanese Patent Application Publication No. H07-160017 (Patent literature 7). A fourth configuration is that of a single layer-type photoconductor in which a charge generation material, a hole transport material and an electron transport material are dispersed in one same layer, see, for instance, Patent literature 6 and Japanese Patent Application Publication No. H03-256050 (Patent literature 8). The above classification into four types does not take into account the presence or absence of an undercoat layer.
Among the foregoing, the fourth type, i.e., single layer-type photoconductors, is the object of detailed study, while the scope of practical use thereof is ever wider. A major conceivable reason for this is that single layer-type photoconductors have a configuration wherein the electron transport function of an electron transport material, which is inferior in transport capability to the hole transport function of a hole transport material, is complemented by the hole transport material. Although carriers are also generated inside the film of such a single layer-type photoconductor, since the latter is of dispersed type, the carrier generation amount increases, and the electron transport distance decreases with respect to the hole transport distance, with increasing proximity to the vicinity of the surface of the photoconductive layer. Accordingly, it is deemed that the electron transport capability need not be as high as the hole transport capability. Single layer-type photoconductors afford as a result sufficient environmental stability and fatigue characteristic in practice, as compared with photoconductors of the other three types.
In single layer-type photoconductors, one single film fulfils both the functions of carrier generation and carrier transport, and, accordingly, single layer-type photoconductors are advantageous in terms of making it possible to simplify the coating process and affording a high yield rate and process capability. On the other hand, however, single layer-type photoconductors have been problematic on account of reduced content of binder resin, and reduced durability, as a result of increasing, within one same layer, both the amount of hole transport material and electron transport material in order to enhance sensitivity and speed. In consequence, single layer-type photoconductors have limitations in terms of combining both high sensitivity and high speed with high durability.
A further drawback of single layer-type photoconductors has been the drop in glass transition point, and poorer contamination resistance towards a contact member, when the ratio of binder resin is reduced.
The drop in the glass transition point is further exacerbated when a plasticizer in the form of a phenylene compound is added to into the photoconductive layer of single layer-type photoconductor, as a countermeasure against contamination by oils/fats and sebum, as described in Japanese Patent Application Publication No. 2007-163523 (Patent literature 9), Japanese Patent Application Publication No. 2007-256768 (Patent literature 10), and Japanese Patent Application Publication No. 2007-121733 (Patent literature 11). This has resulted in problems of significant creep deformation, and manifestation of printing defects, in equipment with high contact pressure of rollers or the like that are in contact with organic photoconductors.
It is thus difficult to achieve concurrently sensitivity, durability, and contamination resistance, by using conventional single layer-type positively-chargeable organic photoconductors, in coping with ever smaller sizes, higher speeds, higher resolutions and colorization in equipment in recent years. Thus novel multilayer-type positively-chargeable photoconductors have been proposed that are a sequential stack of a charge transport layer and a charge generation layer, see, for instance, Japanese Patent Application Publication No. 2009-288569 (Patent literature 12) and WO 2009/104571 (Patent literature 13). The layer configuration of multilayer-type positively-chargeable photoconductors is similar to the layer configuration of the above-described first type, but herein the amount of charge generation material comprised in the charge generation layer is reduced, the electron transport material is incorporated, the thickness of the photoconductor can be brought close to that of the low-layer charge transport layer, and, moreover, the addition amount of hole transport material inside the charge generation layer can be reduced. It becomes accordingly possible to set a higher resin ratio within the charge generation layer than in the case of conventional single layer-type photoconductors, and to achieve both higher sensitivity and higher durability.
However, the durability against sebum contamination in both the multilayer-type positively-chargeable organic photoconductors and single layer-type photoconductors is not necessarily sufficient, and surface cracks, as well as image defects such as white spots and black spots occur in some instances when human nose fat, or scalp sebum, remains adhered to the surface of the photoconductor over long periods of time.
Known conventional technologies pertaining to improvements in photoconductors include, in addition to those above, also a technology that involves using polymer microparticles in the form of microspheres, of core-shell type, that have, on the outer peripheral section, a functional layer made up of functional groups having a charge generation function, and in the interior, an adsorption layer having enough charge as to enable adsorption on account of electrostatic interactions Japanese Patent Application Publication No. 2003-228184 (Patent literature 14), and a technology that involves using a cured product of an oligomer with a radically polymerizable compound having a charge-transporting structural moiety, wherein the oligomer has a hyperbranched structure or a dendrimer structure having at least an acryloyloxy group or a methacryloyloxy group at the termini, see Japanese Patent Application Publication No. 2010-276699 (Patent literature 15). Further known technologies include a technology that involves incorporating, into the surface layer of a photoconductor, a binder resin and a linear vinyl polymer having long-chain alkyl groups in side chains, see Japanese Patent Application Publication No. 2003-255580 (Patent literature 16), and a technology that involves enhancing crosslinking and surface lubricity of a protective layer of a photoconductor, by using, as the protective layer, a layer made up of a cured resin that is obtained by polymerizing a radically polymerizable monomer in the presence of a mercapto-modified silicone oil, see Japanese Patent Application Publication No. 2012-93403 (Patent literature 17).
As described above, although it is possible to achieve resistance towards contamination by oils/fats such as grease, concurrently with high sensitivity/high-speed combined with high durability, both in positively-chargeable organic photoconductors of single layer type and in positively-chargeable organic photoconductors of multilayer type, such as those disclosed in Patent literature 12 and 13, no photoconductor has been heretofore capable of preventing completely the occurrence of image defects derived from contamination i.e. derived from the occurrence of cracks by human sebum adhesion.
Therefore, it is an object of the present invention to solve the above problems by providing an electrophotographic photoconductor of high sensitivity and fast response, as well as high durability, that is used in high-resolution, high-speed electrophotographic apparatuses of positive charging schemes, such that the electrophotographic photoconductor boasts superior operational stability and affords stably high image quality, without the occurrence of problems with image memory, a contact member, or image defects due to cracks caused by contamination by oils/fats or sebum, and to provide a method for producing the electrophotographic photoconductor, and an electrophotographic apparatus.